Programmable gain amplifiers are very important in wireless communication system (such as GSM) requiring highly dynamic range. Therefore, a programmable gain amplifier with a larger gain controllable range and highly controllable linearity is beneficial to the whole effect of the system and complexity of the system. The gain control of a programmable gain amplifier can be achieved by changing the direct bias current (change gm of a transistor) and load resistance of an amplifier, wherein the maximum gain change is to control the bias current.
FIG. 1 shows a programmable gain amplifier with a varied gain range in the prior art design (U.S. Pat. No. 4,394,777), wherein the programmable gain amplifier includes a reference current generating circuit, a plurality of resistor elements, an auxiliary reference voltage generator, a plurality of switches which are connected to the resistor elements and a voltage to current converter. In the current gain control method, OP feedback circuit serves to control the depth of a MOS switch for adjusting the direct bias current of an amplifier or using fixed voltage to switch load resistor for change the value of current. While the abovesaid method has the following disadvantages: 1. The OP feedback circuit has a problem of stability. 2. It is difficult to control the depth of MOS switch. 3. The fixing voltage is used in the MOS manufacturing process, the variation of V.sub.T causes that the performance of the circuit can not be controlled. 3. The layout of the resistors occupys a large area and the error in manufacturing process is large. 4. The structure of the circuit is complex. 5. Extra power consumption is required.